1. Field of the Invention
The present invention generally relates to scanning projection and display systems. More particularly, it relates to projection and display systems that scan one or more modulated laser beams to generate an image.
2. Description of Related Art
Rapid advances in information processing and video production have made it technically possible to provide high resolution video images. However, the quality of the video images seen by a viewer is severely limited by the systems that project the image on a screen. The lack of suitable display systems has been a significant factor in preventing widespread application of high resolution video projectors. Generally, the available projection display systems are either very expensive, or they do not provide a high quality image at high frame rates and high resolution, and with sufficient brightness. If available at an affordable cost, high resolution projection displays would have widespread and diverse uses such as projection TV for consumers and businesses, projectors for meeting room and auditoriums, flight simulators for military uses, and movie projectors for theatres.
Two basic principles, direct emission and light modulation, are commonly used in video projectors. Direct-emission projectors emit their own light. The most common direct emission device is the CRT projector used in home TV projectors. High power versions of CRT projectors have been used for large screen industrial use. In a CRT projector, an electron beam is appropriately modulated to excite phosphors, which in turn generates the color of each pixel in the image that is projected onto a screen.
To display a video (i.e. moving) image, a sequence of frames are displayed very rapidly on the CRT screen. Each frame must be fully scanned by a single electron beam within a very short time period. For example, for a 60 Hz frame refresh rate, each frame must be scanned in less than {fraction (1/60)} of a second. Because a frame is defined by a number of adjacent lines, each line must be scanned within a small fraction of the frame period, depending upon the size of the display. For example, in a standard 640xc3x97480 pixel format, each of 480 lines must be scanned in less than 3.5 microseconds (i.e. 3.5xc3x9710xe2x88x926 second). Of course, larger pixel formats (e.g. 800xc3x97600 or 2000xc3x971000 have a greater number of pixels and lines, and therefore such larger formats require correspondingly faster line scans.
Despite the large number of lines that are scanned, direct-emission projectors have a relatively straightforward design that conceptually consists of only a controllable light source and optics. The evolution of the CRT projector to its present state illustrates how, owing to its inherent simplicity, a direct-emission display mechanism can be readily produced and later scaled-up to higher brightness and resolution levels. However, as resolution and brightness requirements increase, CRT-based projectors reach some physical limits and therefore, other ways to project video and computer information have been proposed and developed.
Light-modulation projectors have been proposed in which red, green, and blues lasers are individually modulated and combined to generate a full color image on a projection screen. In light-modulation projectors, laser radiation is modulated in a modulator array that switches individual display elements (pixels) on or off. Liquid crystal display (LCD) panels are common light modulators. Other modulators, such as acousto-optic modulators, oil film modulators, and deformable micro-mirrors (DMDs), are also available.
Light modulation projectors can be classified depending upon whether the modulated beam is scanned or not. Generally, a non-scanning projector requires a two-dimensional modulator array large enough to have a one-to-one relationship with the screen pixels, and each pixel is constantly illuminated. For example each array pixel in a 640xc3x97480 DMD array is directly mapped onto a screen pixel. However, non-scanning systems have demanding electronics requirements due to the large number of pixels that must be accessed simultaneously, and the available modulators are difficult and expensive to scale up to such sizes. Unfortunately, in the context of large high resolution displays, the available two-dimensional modulators have a limited pixel count. As a point of reference, present-day full-color LCD-based and DMD-based display systems are limited to pixel counts of less than 106, while some displays require pixel counts of 2xc3x97106 and greater. Furthermore, the brightness of the projected display in constant illumination systems is low at higher pixel counts because the fractional area of the pixels decreases at higher densities, thereby reducing light output. The design issues associated with such constant illumination systems are difficult and complex, and in many cases they directly limit the achievable results.
In order to overcome the limitations and problems inherent in non-scanning projection systems that constantly illuminate each pixel, scanning architectures have been developed in which multiple laser beams are very rapidly scanned across a screen to create an image. The concept of scanning a single modulated laser beam is similar to the concept of scanning an electron beam in the familiar CRT monitor widely used for computers and TVs. However, a practical system for scanning an electron beam is very different from a practical system for scanning a modulated laser beam. Laser scanning configurations have practical hardware limitations on the modulation rate and the scanning rate. Particularly, the available light modulators are limited in bandwidth. Further bandwidth limitations are imposed by the scanner and scanning configuration. A typical two-dimensional (2D) scanner uses a polygon scanner for horizontal scanning and a galvanometer-actuated mirror (a xe2x80x9cgalvo mirrorxe2x80x9d) for vertical scanning, both of which have limited bandwidth (i.e. scanning rate). Although some high bandwidth polygon scanners are available, they are very expensive and impractical. Galvo mirrors are much less expensive, but they have much smaller bandwidth, and are plagued with nonlinearities.
To address the limitations of scanning speed and modulation bandwidth, parallel scanning systems have been proposed in which multiple modulated laser beams are scanned simultaneously, thereby reducing the scanning speed and modulation bandwidth requirements, such as disclosed in U.S. Pat. No. 5,534,950 issued Jul. 9, 1996 and PCT publication WO95/10159, published Apr. 13, 1995.
Parallel scanning techniques (architectures) used in prior multi-beam laser display systems can be divided into three basic techniques for purpose of discussion. These three scanning architectures are termed herein: (1) vernier, (2) paintbrush, and (3) pushbroom. In these scanning architectures, the image space can be treated as being divided into bands, which are defined as groups of adjacent lines. Typically, the number of lines in each band is equal throughout the image space, for example one parallel scanning system for a 640xc3x97480 format has 48 bands, each band including ten lines.
In the vernier scan architecture, multiple modulated laser beams simultaneously write multiple bands. Each band is written by a laser beam that is scanned across the screen to write a first line, then deflected to a next line in the band, then scanned to write the next line, and so forth until all the lines in the band have been written. Multiple beams are used to scan multiple bands in parallel; for example, one system may include 64 modulated laser beams that are vertically arranged with a spacing between adjacent beams of about {fraction (1/64)} of the screen height. The 64 beams write 64 bands in parallel in the image space.
In the paintbrush scan architecture, the laser beams are closely aligned in a vertical arrangement so that all lines in one band are written simultaneously. For example if a band has 64 lines, then 64 closely-positioned laser beams are utilized to scan each band. If the image has 640 lines, then a frame in an image is written by scanning ten bands in sequence, one after the other.
The pushbroom scan architecture is a variation of the paintbrush system in which the number of laser beams is equal to the number of lines in the image space. For example, a pushbroom system for a 1000 line image requires 1000 laser beams, which reduces the bandwidth requirements of the modulators at the cost of a need for additional modulators. One advantage of the pushbroom system is that a frame is displayed in the time it takes to write one line due to the parallel writing of each line in the image.
A significant challenge in implementing laser scanning systems relates to achieving a clear, high quality image with high resolution while avoiding psycho-visual artifacts of laser scanning such as latency, image doubling, and image breakup. Particularly, in the context of producing moving images, it is important to reduce scan-related artifacts that affect the image. One problem with laser scanning systems relates to the differential in refresh time between adjacent or neighboring pixels. If objects traverse the image at high angular rates, each pixel of the image must be refreshed at a time instant very close to the refreshment time of all neighboring pixels. In non-interlaced CRT-based systems which raster scan each line sequentially, the maximum time between writing adjacent pixels on adjacent lines is small enough that even a quickly traversing object is unlikely to have moved by as much as a single pixel. For example, in a 1024-line non-interlaced format with a 60 Hz refresh rate, the maximum time delay between writing adjacent pixels is 15 microseconds.
However, the laser-based vernier scan architecture discussed previously suffers from severe artifacts relating to the large differential refresh time between neighboring pixels. Particularly, the last line in the first band is written by the first beam at the end of a frame, while the first line in the second band is written by the second beam at the beginning of the same frame. As a consequence, adjacent pixels in the last line of the first band and the first line of the second band are refreshed with an unacceptably large time differential. For example, in the 1024-line non-interlaced format with a 60 Hz refresh rate discussed previously, the time delay between writing adjacent pixels can be up to about 16 milliseconds, which is approximately 1000 times greater than that associated with a non-interlaced CRT raster scan. During this much longer interval, a fast-traversing object may be displaced by many pixels. This circumstance can cause the psycho-visual system of the viewer to observe both image doubling and image breakup, the severity of which depends upon the transverse velocity of the object being viewed and upon the instantaneous location of that object relative to the boundaries that lie between regions of the image that are written by different beams.
The paintbrush scan architecture, in which each band is written in sequence by closely-positioned beams, can reduce the time differential between writing adjacent pixels and accordingly reduce artifacts. Specifically, in the paintbrush architecture the time differential is equal to the time between writing adjacent bands. For example, in the 1024-line non-interlaced format with a 60 Hz refresh rate discussed above, the maximum time delay between writing adjacent pixels would be about 1.0 millisecond, a delay short enough to eliminate some of the effects of psychovisually-induced image artifacts. Unfortunately, implementation of the paintbrush architecture poses severe problems. First, it has proven very difficult to balance the relative brightness of closely-positioned beams in a manner consistent with the elimination of horizontal banding. Furthermore, it has been found that the geometric interfaces between adjacent bands must be matched with high accuracy, which in turn requires extreme precision in both the rate and the linearity of the vertical scanning mechanism. Such an extremely precise scanning mechanism is not presently available.
In a pushbroom architecture, the entire frame is scanned in parallel, and therefore the maximum time delay between adjacent pixels corresponds to the interval required for a column of beamlets to move horizontally by one pixel, which can be very short. Accordingly psycho-visual artifacts are negligible in the pushbroom approach. Unfortunately, implementation of the pushbroom approach requires linear array modulators with thousands of individual elements, none of which are commercially available at the present time. Furthermore, the closely-spaced beams pose the same problem as in the paintbrush approach; particularly it is very difficult to balance the relative brightness between lines, thereby eliminating horizontal banding.
In order to overcome the limitations of the prior art, the present invention provides a progressive scan architecture that can display a high-resolution video image. A two-dimensional image, defined by data arranged in a plurality of rows corresponding to a plurality of lines in the image, is progressively scanned in a system in which at least two beams are scanned alternately, one after the other. Adjacent lines can be scanned in sequence from top to bottom, which advantageously reduces or eliminates psycho-visual effects such as image doubling, breakup problems, and reduces or eliminates latency problems. Furthermore, the top-to-bottom progressive scan architecture is tolerant of non-linearities in the vertical scanner; a feature that permits the use of a low-cost galvanometer while avoiding horizontal banding in the displayed image. High resolution and/or high pixel counts can be achieved by adding beams in a straight forward manner, without requiring a high-speed polygon scanner or a highly linear galvo scanner.
A method for progressively scanning a plurality of laser beams to generate a two-dimensional image includes alternately scanning a first and a second laser beam along a first axis of said two-dimensional image (typically the horizontal axis), and scanning the laser beams at an approximately constant rate along a second axis substantially perpendicular to said first axis (typically the vertical axis). In some embodiments, the progressive scanning method further comprises alternately scanning one or more additional pairs of laser beams with the first and second laser beams. One progressive scanning method includes rotating a polygon mirror arranged to receive the first laser beam along a first optical path and the second laser beam along a second optical path non-parallel to the first optical path. The first and second laser beams define a row along a perimeter of the polygon mirror. Some methods include generating a beam of laser radiation, modulating the beam, and switching (i.e. alternating) the modulated beam between a first and second optical path so that the modulated beam provides the first beam when switched to the first optical path and provides the second beam when switched to the second optical path.
In a system described herein a polygon scanner scans at least two modulated laser beams horizontally and a galvanometer (galvo) mirror scans the beams vertically. The beam spots (the xe2x80x9cincident locationsxe2x80x9d) on the polygon scanner are arranged in a row aligned with the perimeter of the polygon. The beams in the row are arranged in pairs, and typically only one beam from each pair will be scanning at any one time. In such an embodiment, where the duty cycle is slightly less than 50%, the laser illumination can be switched between two interleaved beam scans, thereby allowing a single modulator to be used for both beams, which can provide significant cost advantages and improve system efficiency without requiring a long polygon facet. Additional pairs of beams can be added as described herein to improve resolution and/or increase pixel count without requiring a high-speed polygon scanner or a highly-linear galvo scanner. For example, increasing the number of pairs from one to two can be used to double the resolution or to provide a two-fold reduction in polygon rotational speed. Accordingly, a low-cost polygon scanning mirror, with a small diameter and a small facet length well within the limits of available technology, can be utilized. For example, the height of each facet need be only one beam diameter and its length need only be two beam diameters, which provides significant cost advantages.
A method described herein for progressively scanning a pair of modulated laser beams includes modulating a laser beam with a first row of image data, propagating the modulated laser beam along a first optical path, and scanning the first modulated laser beam to display a first image line during a first time interval that begins at a first time. During a second time interval that begins at a second time delayed from the first time by a uniform time interval, a second laser beam is modulated with a second row of image data, the modulated laser beam is propagated along a second optical path that is displaced from the first optical path, but substantially in the same plane, and then scanned to display a second image line during a second time interval. In subsequent time intervals, each subsequent row of image data is displayed by alternating the first and second laser beams, thereby displaying the lines in the image. Each subsequent line scanning operation begins at a time delayed from the beginning of the preceding line""s scanning operation by the uniform time interval.
A progressive scanning system for scanning the pair of modulated laser beams includes a polygon scanner for scanning the lines of the image along a first axis (typically the horizontal axis) and a scanning mirror for scanning along a second axis (typically the vertical axis). The polygon scanner has a perimeter and a plurality of reflective facets are formed on the perimeter, each of the facets having an approximately equal length. During operation, the polygon scanner rotates about its central axis to sequentially present the reflective facets to a row of modulated laser beams. Particularly, a first optical system directs the first modulated laser beam along a first optical path to a first incident location on the perimeter of the polygon scanner, and a second optical system directs a second modulated laser beam along a second optical path to a second incident location on the perimeter of the polygon scanner, the second incident location being spaced apart from the first incident location. The second optical path is arranged non-parallel to the first optical path so that the second laser beam propagates non-parallel to the first laser beam, although the first and second optical paths may lie in the same plane (i.e., substantially uniplanar but non-parallel). The scanning mirror is arranged to receive the first and second beams from the polygon scanner. The scanning mirror includes a motive system to scan the first and second beams at an approximately constant rate along a second axis approximately perpendicular to the first axis, such that the first modulated laser beam scans a first line in the image and the second modulated laser beam scans a second line delayed from the first line.
In some embodiments two light modulators are used, one for each beam. In one such embodiment a primary laser beam is split by a beamsplitter into a pair of beams having approximately equal powers, and a first beam is modulated by a first light modulator during the first time interval to provide the first modulated laser beam and the second laser beam is modulated by a second light modulator during the second time interval to provide the second modulated laser beam.
In other embodiments a single light modulator is used to modulate the primary laser beam, and an optical switch is used to switch the modulated beam between the first and the second optical paths to provide the first and second laser beams. Particularly, the primary laser beam is modulated by the modulator during a first time interval and scanned to display a first image line, then switched from the first optical path to the second optical path, then modulated during the second time interval and scanned to display the second image line, and then switched from the second optical path to the first optical path after the second time interval to repeat the operations for all lines in the image. Advantageously, such an arrangement reduces by a factor of two the number of high-cost modulators required at the cost of an additional switch. Furthermore, such a system efficiently uses the energy of the primary laser beam.
The progressive scanning system easily can be expanded by addition of one or more pairs of beams that progressively scan the image in concert with the first pair. Generally, additional pairs are added so that the total number of pairs is a power of two; i.e. one pair, two pairs, four pair, eight pairs, and so forth. The spatial separation between the two beams in each additional pair is approximately equal to the spatial separation of the first pair. The first beam of each succeeding pair typically is situated midway between the first and second beams of another pair, and the propagation angle between each adjacent beam is approximately equal, while lying substantially within a single plane. For example, a second pair is arranged with the first pair in a row in which the first beam of the second pair is situated midway between the first and second beams of the first pair. Each incident location is separated by an approximately equal distance from the adjacent incident locations. Furthermore, the propagation of the four beams are non-parallel to each other, having propagation angles between adjacent beams that is approximately uniform, although the four beam paths may lie in a single plane (substantially uniplanar but non-parallel within that plane). In an example with four pairs, a third and fourth pair are arranged so that the first beam of the third pair is situated between the first beams of the first and second pair, and the first beam of the fourth pair is situated between the first beam of the second pair and the second beam of the first pair.
Typically, the first and second incident locations have a center-to-center distance of approximately one-half of the polygon facet length.
In some embodiments, a full-color image can be displayed by the progressive scanning system described herein. For full-color images, each of the beams described can incorporate separate red, green and blue (RGB) components which are individually modulated by separate red, green, and blue modulators. In one such embodiment, the first modulated laser beam comprises a first RGB combined laser beam, and the second modulated laser beam comprises a second RGB combined laser beam that are progressively scanned as described herein. Additional RGB beams can be added as described herein to increase resolution and/or reduce polygon rotational speed.